As a Christian, it upsets me that many argue that God exists by saying that he has to for morality to exist. Though this may be the case, it is not confirmed, nor can it be confirmed.
I will be arguing against the notion that religion is the basis for morality.

My opening argument will explain that morality is relative.

Many different cultures, whether religious or not, have different morals. I know many nonreligious people who support abortion, for example. Their opinions couldn't have come from a religion, because they do not practice a religion.
Another reason I believe this is the law that punishes those who murder. Though yes, the Bible and many other holy texts will condemn murder: "You shalt not murder." Exodus 20:13
But murder in itself most likely was regarded as wrong due to the fact that no-one wants to die. You wouldn't want to be killed, so why allow it?
Also, if murder were legal, it would be much harder to maintain a civilization and to have fair democracy or competition.
Feel free to refute this argument. I am excited to see what people say.